When you think of Spain, places like Barcelona and its world-renowned Gaudí-spawned edifices, like Sagrada Família or Casa Batlló, Madrid and its grand boulevards and lush El Retiro Park, or even Mallorca’s picturesque calas and medieval villages, probably come to mind. That’s not for nothing: these 3 popular spots are Spain’s easiest destinations to reach if you’re flying from the United …